Attenuation fin blanket for a feed horn

Abstract: A feed horn (100) includes a top side (114) and a bottom side (116). A left side (118) connects one edge of the top side (114) with one edge of the bottom side (116). A right side (120) connects an opposite edge of the top side (114) with an opposite edge of the bottom (116). The improved feed horn (100) includes an attenuation device (130, 199) connected to an outer surface of at least one of the top and bottom sides (114, 116) for absorbing microwave energy of far-out side lobes. The attenuation means (130, 199) includes a plurality of fins (134, 200, 202). The top and bottom sides are normal to an electric field (E) to be radiated.

Description: BACKGROUND OF THE INVENTION 1. Technical Field This invention relates to feed horns and, more particularly, to feed horns including an attenuation fin blanket. 2. Discussion Horn antennas or feed horns have been widely used as a feed element for large radioastronomy, satellite tracking and communications dishes which primarily operate using microwave frequencies. In addition to the utility of the feed horn as a feed for reflectors and lenses, the feed horn is a common element of phased arrays and serves as a universal standard for calibration and gain measurement. Radio Engineering and Electronic physics, Vol. 25, No. 12, December 1980, Washington, U.S., discloses a feed horn and attenuation means connected to an outer surface of at least one of the top and bottom sides of the horn, including a plurality of fins. The total field radiated by the feed horn is a combination of a direct field and diffractions from edges of an aperture of the feed horn. Edge diffractions, particularly those occurring at edges which are normal to an electric field, influence the radiation pattern of the antenna. The diffractions generate microwave energy in main (or on-axis) lobes, near lobes, far-out side lobes and back lobes. However, the microwave energy generated by the diffractions in the far-out side lobes and back lobes has a much greater and undesirable effect. By reducing far-out side lobes and back lobes, the effects of radio frequency interference can be reduced significantly. Optimum feed horns maximize the main (or on-axis) lobe and minimize far-out side and back lobes. Conventional methods of reducing the effects of diffraction include corrugations on the inside surface of the feed horn, curving the walls of the feed horn near the aperture, and connecting a C-shaped metal attachment to the outer surface of the feed horn. However, such prior approaches have been undesirable due to increased weight which can be critical in satellite applications. Attenuation of the far-out side and back lobes has been insufficient to meet commercial needs for reduced radio frequency interference. In addition, feed horns typically require an aperture cover to provide thermal stability and to prevent contamination. However, fitting the aperture cover over the aperture of feed horns with curved walls has proven to be difficult. Therefore, it is desirable to design a lightweight feed horn producing radiation patterns with highly attenuated far-out side and back lobes without significant weight increase. According to the present invention there is provided a feed horn that absorbs microwave energy comprising a top side, a bottom side, a left side connecting one edge of said top side with one edge of said bottom side, a right side connecting an opposite edge of said top side with an opposite edge of said bottom side, and attenuation means, connected to an outer surface of at least one of said top and bottom sides, for absorbing microwave energy of far-out side and back lobes and including a plurality of fins, characterised in that the fins have a height (H) greater than or equal to one-half of an operating wavelength. According to another feature of the invention, each fin is triangular and includes a crest. The crest of each of the plurality of fins are substantially parallel to each other. The crest of the fins has a length greater than an operating wavelength. According to another feature of the invention, each fin includes a crest which is spaced from an adjacent crest of an adjacent fin by a distance less than or equal to one-half of the operating wavelength. According to still another feature of the invention, the top, bottom, left and right sides are flared to form a pyramidal feed horn. The attenuation device has a parallelogram cross-section. The attenuation device includes at least one triangular surface connecting adjacent fins. Other objects, features and advantages will be readily apparent. BRIEF DESCRIPTION OF THE DRAWINGS The various advantages of the present invention will become apparent to those skilled in the art after studying the following specification and by reference to the drawings in which: FIG. 1A is an end view of a prior art feed horn incorporating a C-shaped attenuation element; FIG. 1B is a side view of the prior art feed horn of FIG. 1A; FIG. 2A is an end view of a prior art feed horn incorporating curved walls; FIG. 2B is side view of the prior art feed horn of FIG. 2A; FIG. 3A is a perspective view of an attenuation fin blanket according to the present invention; FIG. 3B is a perspective view of the attenuation fin blanket attached to a pyramidal feed horn; FIG. 3C is a side view of the attenuation fin blanket and feed horn of FIG. 3B; FIG. 4 illustrates elevation side lobe reduction of the feed horn incorporating the attenuation fin blanket; FIG. 5 illustrates azimuth side lobe reduction of the feed horn incorporating the attenuation fin blanket; FIG. 6 is a perspective view of an alternate feed horn including vertical fins; and FIG. 7 is a perspective view of a feed horn array including vertical fins. DESCRIPTION OF THE PREFERRED EMBODIMENT Antennas convert electrical energy to electromagnetic waves that radiate away from the antenna at speeds near the speed of light and also convert electromagnetic waves back to electrical energy. Radiation of electromagnetic energy is based on the principle that a moving electric field creates a magnetic field and a moving magnetic field creates an electric field. The electric (E) and magnetic (H) fields together form electromagnetic radiation and are perpendicular to each other and their direction of motion. Antennas normally radiate in all directions. Feed horns are used to direct transmitted electromagnetic radiation and to receive electromagnetic radiation from a particular direction. Referring to FIGS. 1A and 1B, a first prior art pyramidal feed horn 10 includes a base 12 which can be connected to a waveguide or directly to a radiating element, top and bottom flaring sides 14 and 16, and left and right flaring sides 18 and 20. The top and bottom flaring sides 14 and 16 and the left and right flaring sides 18 and 20 define a pyramid-shape with a small aperture 22 adjacent the base 12 and a large aperture 24 at an opposite end. The designations of top, bottom, left and right are made when looking through the large aperture 24 towards the small aperture 22. C-shaped metal attachments 30 and 32 are connected to the top and bottom flaring sides 14 and 16, respectively, to redirect microwave energy radiating from a back portion of the feed horn 10 to side portions of the feed horn 10. Specifically, the C-shaped metal attachments 30 and 32 directly reduce back lobes and far-out side lobes of the radiation pattern. A length "L" of the C-shaped attachments 30 and 32 is typically normal to a direction of the E field. In other words, the feed horn 10 is excited such that the E field propagates in a plane substantially (due to flaring of the sides) parallel to the left and right flaring sides 18 and 20 and the H field propagates in a plane substantially parallel to the top and bottom flaring sides 18 and 20. In FIGS. 2A and 2B, a second prior art feed horn 40 includes a base 42, top and bottom flaring sides 44 and 46, and left and right flaring sides 48 and 50. The top and bottom flaring sides 44 and 46 and the left and right flaring sides 48 and 50 define a pyramid-shape with a small aperture 52 adjacent the base 42 and a large aperture 54 at an opposite end. As best seen in FIG. 2A, the top and bottom flaring sides 44 and 46 are curved inwardly towards each other. Similarly, the left and right flaring sides 48 and 50 are curved inwardly towards each other. The curved sides of the second feed horn 40 shift far-out side lobes into near side lobes. Referring to FIGS. 3A, 3B and 3C, a feed horn 100 according to the invention includes a base 112, top and bottom sides 114 and 116, and left and right sides 118 and 120. The top, bottom, left and right sides 114, 116, 118, and 120 can be flared as with a pyramidal feed horn. The top and bottom flaring sides 114 and 116 and the left and right flaring sides 118 and 120 can define a pyramid-shape with a small aperture 122 adjacent the base 112 and a large aperture 124 at an opposite end. As with FIGS. 1A and 1B, the designations of top, bottom, left and right are made when looking through the large aperture 124 towards the small aperture 122. The feed horn 100 is positioned such that an E field (identified by arrow "E" in FIG. 3B) propagates in a plane substantially (due to flaring) parallel to the left and right flaring sides 118 and 120 and the H field propagates in a plane substantially parallel to the top and bottom flaring sides 118 and 120. An attenuation fin blanket 130 includes a plurality of fins 134 each including first and second sides 135 and 136. The first side 135 of one fin 134 is connected to the second side 136 of an adjacent fin 134 by a rectangular surface 138 which provides uniform spacing for the fins along the flaring top surface 114 of the feed horn 100. A crest 140 of the fins 134 is preferably spaced a distance "D" which is less than one-half of an operating wavelength (corresponding to a design frequency). A height of the fins "H" is preferably greater than one-half of the operating wavelength. The attenuation fin blanket 130 can be attached to an outer side of the top surface 114 which is perpendicular to the E field designated by the arrow "E" adjacent the large aperture 124 by any suitable means, for example fasteners, adhesive, etc. The attenuation fin blanket 130 has a length "L" preferably greater than the operating wavelength. When viewed as in FIG. 3C, the fins have a parallelogram cross-section. A top ridge 148 (FIG. 3C) defined by the crests 140 of the fins 134 should be substantially parallel to the top surface 114. Preferably, two attenuation fin blankets 130 5 and 130' (not shown) are attached to one feed horn 100. One attenuation fin blanket 130 is attached as shown in FIG. 3, and another attenuation fin blanket 130 can be attached to an outer surface of the flaring bottom side 116 in an analogous manner. As can be appreciated, the attenuation fin blanket 130 is attached to the bottom side 116 which is also normal to the E field indicated by the arrow "E". The attenuation fin blanket 130 should be made of a material which absorbs microwave energy. For example, the attenuation fin blanket can be made of microwave absorbing film, such as polyimide resin film which provides both thermal insulation and resistance. Preferably, the polyimide resin film is impregnated with carbon particles. Polyimide resin film and carbon-impregnated polyimide resin film are available from DuPont, Inc. under the trademark KAPTON®. Another microwave-absorbing material suitable for the attenuation fin blanket 130 is fiberglass covered with a metallic film. The metallic film can be nickel, chrome, or an alloy of nickel and chrome. Preferably, the fiberglass is 20-30 mm. thick. Other suitable materials for the attenuation fin blanket will be readily apparent. The material used for the attenuation fin blanket 130 should have a resistance less than 600Ω/square. A resistance of approximately 200Ω/square is preferable. Portions of the outer surfaces of the left, right, top and bottom sides 114, 116, 118 and 120 can be covered with a thin cover 149 having a much higher resistance than the material used for the attenuation fin blanket 130 for thermal insulation purposes. Metallized film or vacuum deposited aluminum (VDA) layers can also be used. Alternately, the thin cover 149 can be adhered to the outer surfaces not covered by the attenuation fin blanket 130. The thin cover 149 should have a resistance on the order of 10 4 Ω/square or higher, for example 10 6 Ω/square. FIG. 4 illustrates elevation side lobe reduction and FIG. 5 illustrates azimuth side lobe reduction of the feed horn 100 of the present invention. A radiation pattern 150 was generated by a conventional feed horn without the attenuation fin blanket 130 of the present invention and a radiation pattern 160 was generated by the feed horn 100 with the attenuation fin blanket 130 affixed to the flaring top and bottom sides 114 and 116 according to the invention. A first length of the flaring top and bottom sides 114 and 116 at the large aperture 154 is 9.0" and a second length of the flaring left and right sides 118 and 120 at the large aperture is 8.7". The feed horn 100 was operated at 3.95 GHz. Table A summarizes the results: Angle from Boresight Azimuth Side lobe Reduction Elevation Side lobe Reduction 90 > 5.0 dB > 2.0 dB 135 > 5.0 dB > 6.0 dB 180 > 20.0 dB > 18.0 dB Similar attenuation was obtained at 3.7 GHz and 4.25 GHz. As can be appreciated, the attenuation fin blanket 130 also provides significant back lobe reduction. In FIG. 6, an alternate feed horn 198 includes an attenuation device 199 with a plurality of vertical fins 200 attached to the top side 114 and an attenuation device 199' with a plurality of fins 202 attached to the bottom side 116 normal to the E-field. For clarity purposes, reference numbers from FIG. 3 will be used where appropriate. The vertical fins can be individually supported or can be formed integrally with a blanket. The vertical fins 200 and 202 have a height greater than or equal to one half wavelength and are spaced less than or equal to one half wavelength. As can be appreciated, the vertical fins 200 and 202 absorb microwave energy behind and to the sides of the feed horn 198 to reduce radio frequency interference. In FIG. 7, a feed horn array 204 includes first and second feed horns 206 and 208 stacked in a direction parallel to the E-field. For clarity purposes, reference numbers from FIG. 3 and 6 will be used where appropriate. The vertical fins 200 are attached to the top surface 114 of the first feed horn 206. The vertical fins 202 are attached to the bottom surface 116 of the second feed horn 208. Trapezoid-shaped sections 210 of microwave absorbing material are connected between corners 212 formed by the bottom side 116 and the left and right sides 118 and 120 of the first feed horn 206 and corners 213 formed by the top side 114 and the left and right sides 118 and 120 of the second feed horn. A roll 214 of microwave absorbing material is located between the bottom side 116 of the first feed horn 206 and the top side 114 of the second feed horn 208. As can be appreciated, the vertical fins 200 and 202, the trapezoid-shaped sections 210, and the roll 214 can be made with microwave absorbing materials described above with respect to attenuation fin blanket 130. Each vertical fin 200 and 202 preferably has a resistance between 160-330 Ω/square. While the fins have been described above in conjunction with pyramidal horn antennas used at microwave frequencies, one skilled in the art can readily adapt the fins for use with other types of feed horns and at other frequencies by scaling the length and height of the fins and the distance between the fins. As can be appreciated, while the feed horn array 204 is shown with vertical fins, individually supported triangular fins or the attenuation fin blanket may also be used. The fins are lightweight, thermally stable and easy to assemble on the feed horns. The fins effectively absorb microwave energy behind and to the sides of the feed horn to reduce radio frequency interference.

Claims: A feed horn (100) that absorbs microwave energy comprising a top side (114), a bottom side (116), a left side (118) connecting one edge of said top side (114) with one edge of said bottom side (116), a right side (120) connecting an opposite edge of said top side (114) with an opposite edge of said bottom side (116), and attenuation means (130,199), connected to an outer surface of at least one of said top and bottom sides (114,116), for absorbing microwave energy of far-out side and back lobes and including a plurality of fins (134,200,202), characterised in that the fins (134,200,202) have a height (H) greater than or equal to one-half of an operating wavelength. The feed horn of Claim 1 wherein said top and bottom sides (114,116) are normal to an electric field (E) to be radiated. The feed horn of Claim 1 wherein each fin (134) is triangular and includes a crest (140), and wherein the crests (140) of each of the plurality of fins (134) are substantially parallel to each other. The feed horn of Claim 3 wherein the crests (140) of the fins (134) have a length (L) greater than or equal to an operating wavelength. The feed horn of Claim 1 wherein each fin (134) includes a crest (140) which is spaced from an adjacent crest (140) of an adjacent fin (134) by a distance (D) less than one-half of the operating wavelength. The feed horn of Claim 1 wherein said top, said bottom, said left, and said right sides (114, 116, 118, 120) are flared to form a pyramidal feed horn (100). The feed horn of Claim 6 wherein said fins (134, 200, 202) have a parallelogram cross-section. The feed horn of Claim 7 wherein said attenuation means (130) includes at least one rectangular surface (138) connecting adjacent fins (134). The feed horn of Claim 1 wherein said attenuation means (130, 199) is made at least partially of polyimide resin film. The feed horn of Claim 1 wherein said attenuation means (130, 199) is made at least partially of carbon-impregnated polyimide resin film. The feed horn of Claim 10 wherein said carbon-impregnated polyimide resin film has a resistance less than 600Ω/square. The feed horn of Claim 1 wherein said attenuation means (130, 199) is attached to said outer surface of said one of said top and bottom surfaces (114, 116) adjacent an aperture (124) of said feed horn (100). The feed horn of Claim 1 further including a second attenuation means (130', 199') for absorbing microwave energy of far-out side lobes, wherein said second attenuation means (130', 199') is connected to the other of said top and bottom sides. The feed horn of Claim 10 further including a cover (149) attached to at least part of the outer surfaces of the left, right, top and bottom sides (114, 116, 188, 120) for thermally insulating the feed horn (100). The feed horn of Claim 14 wherein the cover (149) is made at least partially of carbon-impregnated polyimide resin film having a resistance greater than 10 4 Ω/square. The feed horn of Claim 2 wherein said fins (200, 202) are vertical fins (200, 202) having a height greater than or equal to one-half an operating wavelength and spaced less than or equal to one-half of the operating wavelength. The feed horn of Claim 1 wherein said attenuation means (130, 199) is made at least partially of fiberglass having a metallic film deposited thereon, and wherein said metallic film includes at least one of nickel and chrome.

Inventors: FAHERTY WILLIAM C, GAWLAS GARY J, REYNOLDS ROBERT L, WONG SAMUEL S, FAHERTY, WILLIAM C., GAWLAS, GARY J., REYNOLDS, ROBERT L., WONG, SAMUEL S.
Applicants: HUGHES ELECTRONICS CORP, HUGHES ELECTRONICS CORPORATION
Application year: 1994
Main classification: H01Q17
IPC codes: H01Q17